Some engine control strategies may adjust engine output using feedback control based on detection of various operating parameters. Furthermore, different modes of engine operation may be controlled according to different engine operating parameters. For example, in a lean mode of engine operation, the engine can be controlled according to feedback of the engine air-fuel ratio as detected by an exhaust gas sensor. However, if the sensor degrades, engine operation may be affected potentially resulting in greater errors in engine torque estimation and control, increased emissions and reduced fuel economy. Some engine control strategies advantageously monitor the functionality of sensors.
One approach attempts to monitor sensor degradation and disables the mode upon degradation detection. For example, if it is detected that a sensor has degraded which is used for engine control during a lean operating mode, then the lean operating mode is disabled. Alternatively, if the sensor is not used in control of the operating mode, then the mode may remain enabled. See, for example, U.S. Pat. No. 6,758,185.
However, the inventors herein have recognized that this approach may have some disadvantages. In particular, the above approach may result in un-matched air and fuel amounts in cylinders upon reactivation or deactivation when disabling a mode. For example, disabling a lean or fuel injector cut-out mode of engine operation under increased air load conditions may potentially cause greater errors in engine torque estimation and control, increased emissions and/or reduced drivability as fuel may be provided before excess air is reduced.
The above issues may be addressed by, in one example, a multi-cylinder group engine system operable in at least a first mode and a second mode, where in the first mode a first and second cylinder group combust air and fuel with a lean air-fuel ratio, and where in the second mode at least one of the first and second cylinder groups combusts air and fuel and the other one of the first and second cylinder groups pumps air without injected fuel, the engine system comprising: a fuel injection activity sensor coupled to each cylinder in the first and second cylinder groups; a exhaust gas sensor disposed in an exhaust passage to measure air fuel exhausted from the engine; and a controller configured to transition out of the first mode responsive to detection of exhaust gas sensor degradation and to transition out of the second mode responsive to detection of fuel injection sensor degradation. In one example, the transition out of the second mode may be slower than the transition out of the first mode in response to the respective degradation.
In this way, an engine control strategy may be reconfigured in an efficient manner to avoid modes of operation that use feedback control from degraded sensors. In particular, this approach allows for delayed exit from a degraded mode in order to meet driver demand and reduce the likelihood of decreased drivability and increased emissions. In one example, when operating in a mode with one cylinder group pumping air without injecting fuel, increased air flow overall may be provided to both cylinder groups to meet torque demand. However, if degradation is detected and a mode transition occurs where fuel injection is reactivated to the cut-out cylinder group, un-matched air and fuel amounts may be provided as it may take several cycles to reduce overall airflow in a cylinder. Therefore, a mode transition in response to sensor degradation may be slower than a mode transition in response to a driver request in order to provide a period to match air and fuel amounts, thus reducing the likelihood of errors in emissions and/or engine torque control due to un-matched air and fuel amounts.